The present invention relates to an optical disk having information stored in high density in the form of a mark train such as a pit train, and also a method of manufacturing the optical disk by using a master.
An optical disk is a recording medium on which information has been recorded in the form of a mark train, such as a pit train. The information can be read by applying a light beam onto the recording surface of the optical disk. The optical disk attracts much attention because it can work as a memory for storing a great amount of information in high density. Actually it has been used as a memory in various data-processing systems such as personal computers, and also as a medium recording audio and video signals. Research and development are undergoing to provide an optical disk which can store more information in higher density.
How optical disks are manufactured will be briefly described. First, the master is plated. Using the master thus plated, a stamper is made. Next, the stamper is used, fabricating a number of disk substrates by means of injection molding. A reflecting film made of, for example, aluminum is formed on each disk substrate, and a recording film such as a phase-changing film or a photomagnetic film is formed on the reflecting film. An optical disk is thereby manufactured. Generally, the master is manufactured as follows. First, photoresist is coated on a glass substrate. Then, a laser beam focused by an objective lens is applied to the photoresist. While being applied to the photoresist, the beam has its power modulated in accordance with signals representing the information which is to be recorded on the optical disk. The photoresist exposed to the laser beam is developed with an etchant, whereby pit trains representing the information are formed in the photoresist. The master is thereby manufactured. Alternatively, the light beam may be continuously applied to the photoresist, maintaining a constant power, and the photoresist may be developed with an etchant. In this case, a groove will be formed in the photoresist.
Any optical disk made by using a stamper fabricated by using a master which has a pit train has an identical pit train in the surface of its reflecting film. On the other hand, any optical disk made by using a stamper prepared by using a master which has a groove has a groove in the surface of its recording film. The groove serves as a tracking guide. An exposure light beam may be applied to the recording film, along the groove, forming a mark train (e.g., pits), thus recording information on the recording film of the optical disk.
In order to reproduce information from an optical disk on which the information has been thus recorded, a reproducing light beam is applied to the optical disk. A photodetector detects the beams reflected from the pits and converts the beams into information signals. Hitherto used as the reproducing beam is one emitted from a red-emitting laser. An objective lens focuses the red laser beam. The laser beam is applied to the optical disk, forming thereon a beam spot having a diameter of about 1 .mu.m. In this case, it is possible to reproduce the information which is recorded on the optical disk in the form of pits having a width of about 0.3 to 0.4 .mu.m.
To record information in high density on an optical disk, pits must be formed in the surface of the optical disk in smaller size at shorter intervals between the pits. To reproduce the information recorded in high density, that is, in the form of small pits at short intervals, the reproducing beam must have a small diameter. To achieve these requirements, short-wave lasers, such as an SHG laser element and a blue laser element, are being developed. It is expected that beams emitted from these short-wave lasers can be focused to have a diameter of sub-micron order.
An optimal relationship exists between the diameter of a beam spot and the pit length formed in the surface of an optical disk. Signals having a maximum amplitude will be generated if the pit length is larger than the diameter of the beam spot, provided that the pits have a width about one third the diameter of the beam spot. A blue laser beam can be focused to have its diameter reduced to about 0.6 .mu.m, depending on the optical characteristics of the objective lens used. To reproduce information signals having a maximum amplitude, the pits formed in the surface of the optical disk need to have their width decreased to about 0.2 .mu.m.
As indicated above, the pits in the surface of an optical disk must be small to record information in high density on the optical disk. However, if the pits are arranged at intervals shorter than the diameter of the spot the reproducing beam forms on the surface of the disk, the MTF (Modulation Transfer Function) will decrease. This inevitably reduces the amplitude of the signals reproduced from the optical disk. The shortest pit formed in the disk surface may have the same length approximately equal to or less than the diameter of the beam spot. It may therefore be narrower than any longer pit. If so, the signal generated from the beam reflected from the shortest pit will be extremely reduced.
As described above, small pits may be formed to record information in high density on an optical disk, and a short-wave reproduction beam having a small diameter may be applied to the disk having pits to reproduce information from the disk. In this case, the signal generated from a beam reflected from the shortest pit has an amplitude far smaller than desired. Having an insufficient amplitude, the signal is likely to have increased jitter or to result in an increase of error rate. Ultimately the signal will greatly lower the reliability of the optical disk system in which the disk is used.